1. Field of the Invention
The present invention relates to the field of electroless plating, and more particularly to the application of electroless plating to the fabrication of interconnect structures in semiconductor devices.
2. Discussion of Related Art
The interconnect lines of integrated circuits may be formed with any highly conductive material such as the conductive metals aluminum, copper, and gold. Copper, in particular, has become widely used to form interconnect lines. But, a drawback to using copper and other similar materials is that diffusion of the metal may occur. The metal atoms may diffuse out of the confines of the interconnect line and into the surrounding materials such as the dielectric. This diffusion may cause shorts and ultimately the failure of devices.
The electromigration of the metal used to form the interconnect lines may be countered by forming a diffusion barrier layer containing cobalt substantially surrounding the interconnect lines or capping the interconnect lines. The cobalt diffusion barrier layer may typically be formed by electroless deposition. Electroless plating is a process for depositing a layer of material onto a surface by the chemical reduction of an electrolytic solution in the absence of an external electric current. The electroless plating is typically performed by immersing the subject to be plated into an electroless bath solution. The electroless bath solution for the plating of a cobalt layer may contain cobalt ions that may be provided by cobalt chloride (CoCl2) or cobalt sulfate (CoSO4), a complexing agent such as citric acid, a buffer agent such as ammonium chloride (NH4Cl) or ammonium sulfate (NH4)2SO4, a reducing agent such as hypophosphite, and a pH adjuster such as tetramethylammonium hydroxide (TMAH). In using such a solution it may be necessary to activate the metal to be plated before applying the electroless solution. The metal may be activated by depositing a noble metal catalytic layer such as platinum or palladium on the metal lines. But, this noble metal catalytic layer may increase the resistance of the interconnect lines and cause current leakage. For example, a palladium noble metal layer may result in a 20%-50% increase in copper interconnect line resistance.
Active agents such as the reducing agents dimethylamine borane (DMAB) and borohydrate may be placed in the electroless plating bath to avoid the use of a noble metal catalytic layer. These active agents are highly reactive and serve to catalyze the deposition of the diffusion barrier onto the metal. But, these activation agents such as DMAB and borohydrate may be so reactive that they can dramatically shorten the effective lifetime of the electroless plating bath and may also cause the formation of large particles in the electroless plating bath that may deposit on the metal. Active agents such as DMAB and borohydrate tend to react with the metal particles in the electroless plating solution, and then those particles react with other particles, and so on, until very large metal particles are formed. These large metallic particles may deposit onto the metal being plated and cause current leakage between the interconnect lines leading to shorts and the ultimate breakdown of a device.